In recent years, there has been a strong demand for making motor vehicles light weight. For this purpose, types of materials for producing the motor vehicles are widely being changed. Especially, the conventional soft steel sheets are positively substituted by high strength steel sheets in the automobile industry, because the abovementioned substitution is very easy and effective for making motor vehicles light weight. Also, it is positively promoted to enhance the quality of the high strength steel sheets.
It is known that the mechanical strength of the steel strip is enhanced at a low cost by adding phosphorus into the steel strip so as to a form a phosphorus solid solution therein. This addition of phosphorus is often used in the production of sheet steel of which excellent toughness is not demanded. The principal technique of the abovementioned enhancement of the mechanical strength of the steel strip is disclosed in Japanese Patent Application Publication No. 50-31090. This type of technology is effective for enhancing only the mechanical strength of the steel sheet. Therefore, the resultant steel sheet having an enhanced mechanical strength is useful for the safety of motor vehicles.
However, in recent years, the motor vehicles are required to exhibit various enhanced properties other than mechanical strength. Therefore, high strength steel strips are demanded to have various enhanced properties in addition to the excellent mechanical strength thereof.
In the production of motor vehicles, sheet steels are mostly used in the production of panels, including outside panels and inside panels, of the vehicles. The steel sheet to be used for making panels is required to exhibit a low yield strength (YS), substantially no yield-point elongation (or Luder's elongation), an excellent stretchability and an excellent deep drawability. Also, it is important that the above-mentioned properties of the sheet steel are not deteriorated by natural aging. Further, it is necessary that the sheet steel exhibits a satisfactory resistance to denting. The term "denting property" refers to an oil canning property of the sheet steel and the intensity of denting property of the panel of the motor vehicle is variable depending on the yield strength and thickness of the sheet steel which has been shaped into the desired panel, painted and baked. When a thin sheet is used in order to make the panels of the motor vehicles light weight, the resultant panels exhibit an unsatisfatory resistance to denting. Therefore, it is advantageous that the panels are made from high strength steel strips which exhibit an excellent resistance to denting. High strength steel strips to be used for the above-mentioned use, should exhibit a low yield strength, a high elongation and substantially no yield-point elongation, an excellent resistance in the above-mentioned properties to natural aging and a superior hardening property in paint-baking procedure.
The paint-baking procedure is carried out at a temperature of at the highest 200.degree. C. The hardening phenomenon of the steel strips at the above-mentioned low paint-baking temperature occur only by the formation of carbon and/or nitrogen atmosphere, or precipitation of carbide and/or nitride.
However, the carbon and nitrogen solid solutions can easily diffuse at room temperature and, therefore, cause the resistance of the steel strip to natural aging to deteriorate. Accordingly, it is necessary to minimize the deterioration in the resistance to aging of the steel strip at room temperature while maintaining the hardening property of the steel strip at the paint-baking temperature at a satisfactory level. The activation energy necessary for diffusion of carbon is larger than that of nitrogen. Therefore, usually, nitrogen is fixed in an aluminium killed steel and a portion of carbon is converted to a carbon solid solution in the steel. However, the upper limit of the equilibrium solubility of the carbon in the steel at room temperature is extremely low. Therefore, it is extremely difficult to over-saturate the steel with the carbon solid solution in an amount necessary to obtain a satisfactory hardening property at the paint-baking temperature, by the usual slow cooling procedure in a conventional box-annealing process. Accordingly, it is advantageous that the steel strip is subjected to a continuous annealing procedure which is effective for retaining the carbon solid solution in an over-saturated condition in the steel strip even after an over-aging procedure is applied to the steel strip.
A prior art concerning a process for producing a phosphorus-added steel strip by using a continuous annealing procedure, is disclosed in Japanese Patent Application Publication No. 5427819 (1979). However, the product of the prior art is not satisfactory in certain properties, as a high strength steel strip useful for motor vehicle panels. For example, the resultant steel strip of the prior art exhibits a poor Lankford's value (r value) which closely relates to the deep drawability of the steel strip. In order to enhance the r value of the steel strip it is necessary to apply a two time--cold rolling--annealing process to the steel strip. For another example, the resultant steel strip of the prior art exhibits a poor resistance to aging at room temperature and, therefore, the aging causes the yield strength of the steel strip to be elevated and the yield-point elongation to increase. Therefore, this type of steel strip of the prior art exhibits a poor formability. Even if the steel strip can be shaped by drawing or pressing, the resultant shaped product exhibits a surface defect, that is a so-called stretcher strain.
It is known that a conventional steel strip having a low content of carbon and a low content of manganese, that is, a conventional low carbon-low manganese steel strip, exhibits a high r value. However, when phosphorus is added to this type of steel strip, the resultant phosphorus-added steel strip exhibits a decreased resistance to brittle fracture during press forming, or after press forming. This feature is so-called planar cracking. The intensity of such the brittle fracture of the steel strip increases with the decrease in the content of carbon and with the increase in the content of phosphorus. Therefore, there is a limit in decreasing the content of carbon in the steel strip. Also, the excessively low content of carbon results in a poor mechanical strength of the steel strip.